Remote communication utilizing wireless equipment typically relies on radio frequency (RF) technology. One application of RF technology is in locating, identifying, and tracking objects, such as animals, inventory, and vehicles. RF identification (RFID) systems have been developed that facilitate monitoring of remote objects.
As shown in FIG. 1, a basic RFID system 10 includes two components: an interrogator or reader 12, and a transponder (commonly called an RF tag) 14. The interrogator 12 and RF tag 14 include respective antennas 16, 18. In operation, the interrogator 12 transmits through its antenna 16 a radio frequency interrogation signal 20 to the antenna 18 of the RF tag 14. In response to receiving the interrogation signal 20, the RF tag 14 produces a modulated response signal 22 that is transmitted back to the interrogator 12 through the tag antenna 18 by a process known as continuous wave backscatter.
The substantial advantage of RFID systems is the non-contact, non-line-of-sight capability of the technology. The interrogator 12 emits the interrogation signal 20 with a range from one inch to one hundred feet or more, depending upon its power output and the radio frequency used. Tags can be read through a variety of substances such as odor, fog, ice, paint, dirt, and other visually and environmentally challenging conditions where bar codes or other optically-read technologies would be useless. RF tags can also be read at high speeds, in most cases responding in less than one hundred milliseconds.
RF tags are divided into three main categories: Beam-powered passive tags, battery-powered semi-passive tags, and active tags. Each operates in different ways.
The beam-powered RFID tag is often referred to as a passive device because it derives the energy needed for its operation from the interrogation signal beamed at it. The tag rectifies the field and changes the reflective characteristics of the tag itself, creating a change in reflectivity that is seen at the interrogator. The battery-powered semi-passive RFID tag operates in a similar fashion, modulating its RF cross-section in order to reflect a delta to the interrogator to develop a communication link. Here, the battery is the source of the tag's operational power for optional circuitry. Finally, in the active RF tag, a transmitter is used to create its own radio frequency energy powered by the battery.
The range of communication for such tags varies according to the transmission power of the interrogator 12 and the RF tag 14. Battery-powered tags operating at 2,450 MHz have traditionally been limited to less than ten meters in range. However, devices with sufficient power can reach up to 200 meters in range, depending on the frequency and environmental characteristics.
Security systems have been designed that use RFID devices in providing restricted access to authorized personnel only, i.e., those personnel or employees authorized to carry a tag. However, a drawback to such systems is that tags may be lost or stolen and later used by unauthorized individuals, thus compromising the security. In addition, employees who are no longer authorized to have access or who have been terminated may continue to use the tag or they may tamper with the tag to obtain information stored therein.